An LED light emission device is known, which causes LEDs included in an LED string to emit light by applying a full-wave rectified waveform voltage (hereinafter, called a full-wave rectified voltage) obtained by full-wave rectifying an alternating-current voltage input from a commercial alternating-current power source to the LED string including a plurality of LEDs connected in series. In the LED light emission device, during a period of time in which the full-wave rectified voltage that is applied to the LED string does not reach a threshold voltage at which the LEDs begin to emit light, no current flows through the LED string, and therefore the LEDs included in the LED string do not turn on. When the LED light emission device includes a turned-on period of time in which the LED is turned on and a turned-off period of time in which the LED is turned off, a troublesome phenomenon, such as a flicker and a motion break in which a moving object is seen intermittently, may occur.
An LED drive circuit is known, which eliminates the turned-off period of time or shortens the turned-off period of time by charging a capacitor during the turned-off period of time and causing the capacitor to discharge to turn on LEDs during the period of time in which the full-wave rectified voltage does not reach the light emission threshold voltage of the LED string (for example, see Patent Literature 1).
FIG. 10A is a circuit diagram of an example of a conventional LED drive circuit and FIG. 10B is a circuit diagram of another example of a conventional LED drive circuit.
An LED drive circuit 901 includes a rectifying unit 910, an LED array 912, a first switch 913, a second switch 914, and a switch control unit 915. An LED drive circuit 902 includes a light output compensation unit 911, in addition to the rectifying unit 910, the LED array 912, the first switch 913, the second switch 914, and the switch control unit 915. The light output compensation unit 911 includes capacitors C1 and C2 and diodes D1, D2, and D3, forming a valley-fill circuit. The LED array 912 includes a first LED group 921 and a second LED group 922.
The LED drive circuit 901 supplies a full-wave rectified voltage supplied from the rectifying unit 910 to the LED array 912 and turns on LEDs included in the LED array 912. During a period of time in which the full-wave rectified voltage increases from 0 [V] and reaches a first threshold voltage at which the first LED group 921 begins to emit light, no current flows through the LED array 912, and therefore the LEDs included in the LED array 912 do not turn on. The first threshold voltage is the product of a forward drop voltage of the LEDs connected in series, which are included in the first LED group 921, and the number of serial stages of the LEDs included in the first LED group 921. During a period of time in which the full-wave rectified voltage is higher than or equal to the first threshold voltage and lower than a total threshold voltage, which is the total threshold voltage of the first threshold voltage and a second threshold value at which the second LED group 922 begins to emit light, the first switch 913 turns on and the LEDs included in the first LED group 921 turn on. The second threshold voltage is the product of a forward drop voltage of the LEDs connected in series, which are included in the second LED group 922, and the number of serial stages of the LEDs included in the second LED group 922. When the full-wave rectified voltage is higher than or equal to the total threshold voltage, by the first switch 913 turning off and the second switch 914 turning on, the LEDs included in both the first LED group 921 and the second LED group 922 turn on and all the LEDs included in the LED array 912 turn on. When the full-wave rectified voltage falls, the process is opposite to that when the full-wave rectified voltage increases.
In the LED drive circuit 901, the period of time in which the full-wave rectified voltage is lower than the first threshold voltage is the turned-off period of time in which the LEDs included in the LED array 912 do not turn on.
On the other hand, the LED drive circuit 902 including the light output compensation unit 911 does not have the turned-off period of time. When the full-wave rectified voltage is lower than the first threshold voltage of the first LED group 921, a voltage higher than or equal to the first threshold voltage is applied to the first LED group 921 from the capacitors C1 and C2 and the LEDs included in the first LED group 921 are kept turned on.
The light output compensation unit 911 is explained. The diodes D1, D2, and D3 included in the light output compensation unit 911 turn on when the anode voltage is higher than the cathode voltage and a forward bias is applied, and turn off when the anode voltage is lower than the cathode voltage and a reverse bias is applied. It is assumed that the forward drop voltage of the diodes D1, D2, and D3 is 0 [V] as a matter of convenience and the capacitance of the capacitor C1 is the same as that of C2.
During the period of time in which the full-wave rectified voltage increases from 0 [V] and reaches a peak voltage Vp, the diode D1 turns on and the capacitors C1 and C2 are charged. During the period of time in which the full-wave rectified voltage falls from the peak voltage Vp [V] to a voltage of Vp/2 [V], which is half the peak voltage, the diodes D1, D2, and D3 turn off and the capacitors C1 and C2 enter a floating state. The charged voltage of the capacitors C1 and C2 when the capacitors C1 and C2 enter the floating state is Vp/2 [V]. During the period of time in which the full-wave rectified voltage falls from Vp/2 [V] to 0 [V] and during the period of time in which the full-wave rectified voltage increases from 0 [V] to Vp/2 (V), the diode D1 turns off, the diodes D2 and D3 turn on, and the capacitors C1 and C2 discharge, when the first threshold voltage at which the first LED group 921 begins to emit light is lower than Vp/2 [V], during the period of time in which the full-wave rectified voltage falls from Vp/2 [V] to 0 [V] and during the period of time in which the full-wave rectified voltage increases from 0 [V] to Vp/2 [V], a current is supplied to the first LED group 921 from the capacitors C1 and C2 and the first LED group 921 keeps light emission.
In the LED drive circuit 902, when the full-wave rectified voltage is lower than the first threshold voltage, a current is supplied to the first LED group 921 from the capacitors C1 and C2 included in the light output compensation unit 911 and the LEDs included in the first LED group 921 turn on, and therefore the turned-off period of time may be eliminated.